,BIOD171/BIOD 171 Module 4 Exam | Essential Microbiology
with Lab | Portage Learning | Q & A | 2026/2027 Edition (PDF)
1. Which of the following best defines a pure culture?
A) A culture containing multiple species of microorganisms
B) A culture containing only one type of microorganism
C) A culture that has been stained for identification
D) A culture grown in a liquid medium
Correct Answer: A culture containing only one type of microorganism
Rationale: A pure culture contains a single species of microorganism, allowing for accurate identification
and study. Mixed cultures contain multiple species, which complicates analysis. Staining and liquid
media are unrelated to the definition of purity.
2. What is the primary purpose of a growth medium in microbiology?
A) To inhibit the growth of unwanted microorganisms
B) To distinguish between different species of bacteria
C) To provide nutrients for microbial growth
D) To kill all microorganisms in a sample
Correct Answer: To provide nutrients for microbial growth
Rationale: Growth media are formulated to supply the essential nutrients required for microorganisms
to grow and multiply. Selective media inhibit unwanted organisms, differential media distinguish
between species, and sterilizing agents kill microbes.
3. A researcher is asked to determine which of two vials contains E. coli and which contains Salmonella.
Both are Gram-negative, but only E. coli can ferment lactose. Which type of media would be best suited
for this task?
A) Growth media
B) Selective media
,C) Differential media
D) Enriched media
Correct Answer: Differential media
Rationale: Differential media contain indicators that reveal metabolic differences between species.
MacConkey agar, for example, differentiates lactose fermenters (E. coli, pink colonies) from non-
fermenters (Salmonella, colorless colonies). Selective media inhibit certain microbes, and enriched
media supply extra nutrients.
4. Which of the following statements about LB agar is TRUE?
A) It is a selective medium
B) It is a differential medium
C) It is a non-selective, non-differential medium
D) It is used to isolate Gram-positive bacteria
Correct Answer: It is a non-selective, non-differential medium
Rationale: LB (Luria-Bertani) agar is a general-purpose medium that supports the growth of a wide
variety of microorganisms without restriction. It lacks selective or differential components.
5. True or False: Growth media is best suited for distinguishing between two similar species of bacteria.
A) True
B) False
Correct Answer: False
Rationale: Growth media are designed to simply support microbial growth, not to distinguish between
species. Differential media are specifically formulated to differentiate between closely related
microorganisms based on their metabolic properties.
, 6. What is a fastidious microbe?
A) A microbe that grows rapidly on any medium
B) A microbe with complex growth requirements
C) A microbe that is killed by oxygen
D) A microbe that requires high salt concentrations
Correct Answer: A microbe with complex growth requirements
Rationale: Fastidious microbes have specific and essential nutritional needs that must be met by
enriched media. If these requirements are absent, the organism will not grow.
7. Which of the following is an example of a selective medium?
A) Nutrient agar
B) Blood agar
C) MacConkey agar
D) Tryptic soy agar
Correct Answer: MacConkey agar
Rationale: MacConkey agar contains bile salts and crystal violet, which inhibit the growth of Gram-
positive bacteria while allowing Gram-negative bacteria to grow. This makes it a selective medium. It is
also differential due to its lactose fermentation indicator.
8. What is agar used for in microbiology?
A) As a nutrient source for bacteria
B) To inhibit bacterial growth
C) To provide a solid, smooth surface for microbial growth
D) As a pH indicator
Correct Answer: To provide a solid, smooth surface for microbial growth
with Lab | Portage Learning | Q & A | 2026/2027 Edition (PDF)
1. Which of the following best defines a pure culture?
A) A culture containing multiple species of microorganisms
B) A culture containing only one type of microorganism
C) A culture that has been stained for identification
D) A culture grown in a liquid medium
Correct Answer: A culture containing only one type of microorganism
Rationale: A pure culture contains a single species of microorganism, allowing for accurate identification
and study. Mixed cultures contain multiple species, which complicates analysis. Staining and liquid
media are unrelated to the definition of purity.
2. What is the primary purpose of a growth medium in microbiology?
A) To inhibit the growth of unwanted microorganisms
B) To distinguish between different species of bacteria
C) To provide nutrients for microbial growth
D) To kill all microorganisms in a sample
Correct Answer: To provide nutrients for microbial growth
Rationale: Growth media are formulated to supply the essential nutrients required for microorganisms
to grow and multiply. Selective media inhibit unwanted organisms, differential media distinguish
between species, and sterilizing agents kill microbes.
3. A researcher is asked to determine which of two vials contains E. coli and which contains Salmonella.
Both are Gram-negative, but only E. coli can ferment lactose. Which type of media would be best suited
for this task?
A) Growth media
B) Selective media
,C) Differential media
D) Enriched media
Correct Answer: Differential media
Rationale: Differential media contain indicators that reveal metabolic differences between species.
MacConkey agar, for example, differentiates lactose fermenters (E. coli, pink colonies) from non-
fermenters (Salmonella, colorless colonies). Selective media inhibit certain microbes, and enriched
media supply extra nutrients.
4. Which of the following statements about LB agar is TRUE?
A) It is a selective medium
B) It is a differential medium
C) It is a non-selective, non-differential medium
D) It is used to isolate Gram-positive bacteria
Correct Answer: It is a non-selective, non-differential medium
Rationale: LB (Luria-Bertani) agar is a general-purpose medium that supports the growth of a wide
variety of microorganisms without restriction. It lacks selective or differential components.
5. True or False: Growth media is best suited for distinguishing between two similar species of bacteria.
A) True
B) False
Correct Answer: False
Rationale: Growth media are designed to simply support microbial growth, not to distinguish between
species. Differential media are specifically formulated to differentiate between closely related
microorganisms based on their metabolic properties.
, 6. What is a fastidious microbe?
A) A microbe that grows rapidly on any medium
B) A microbe with complex growth requirements
C) A microbe that is killed by oxygen
D) A microbe that requires high salt concentrations
Correct Answer: A microbe with complex growth requirements
Rationale: Fastidious microbes have specific and essential nutritional needs that must be met by
enriched media. If these requirements are absent, the organism will not grow.
7. Which of the following is an example of a selective medium?
A) Nutrient agar
B) Blood agar
C) MacConkey agar
D) Tryptic soy agar
Correct Answer: MacConkey agar
Rationale: MacConkey agar contains bile salts and crystal violet, which inhibit the growth of Gram-
positive bacteria while allowing Gram-negative bacteria to grow. This makes it a selective medium. It is
also differential due to its lactose fermentation indicator.
8. What is agar used for in microbiology?
A) As a nutrient source for bacteria
B) To inhibit bacterial growth
C) To provide a solid, smooth surface for microbial growth
D) As a pH indicator
Correct Answer: To provide a solid, smooth surface for microbial growth
